Conventional radiation therapy has been shown useful for palliation of hepatic capsular pain, but historically it has played a minor role in the treatment of patients with unresectable liver metastases. This can be attributed chiefly to the low tolerance of the whole liver to radiation. The liver was thought to be a relatively radiosensitive organ and with whole liver irradiation, it was difficult to achieve the radiation doses necessary to eradicate gross tumors without causing radiation induced liver disease. Partial liver irradiation has shown some promise. Limiting irradiation to only one-third of the liver through three-dimensional (3D) conformal radiotherapy techniques, including intensity modulated radiotherapy (IMRT), several studies have shown that partial liver irradiation is safe with doses up to 50-72.6 Gy. However, further dose escalation with conventional radiation therapy techniques risks injury to adjacent abdominal organs.